Alan Johnson, the Education Secretary, has become the clear front-runner in the campaign to succeed John Prescott as Labour’s deputy leader, with a wide range of ministers and MPs publicly declaring their support for him to The Times.

The election will not be officially declared until Mr Prescott resigns, but already six contestants, including four Cabinet ministers, are setting up campaign headquarters, hiring public relations companies, touring the country, lobbying colleagues, giving interviews and commissioning private polls.

Jon Cruddas, the only back-bencher to stand, appears to be winning the support of the unions. He has bagged a £15,000 donation from Amicus, the manufacturing union, and is also expected to be backed by the powerful Transport and General Workers Union.

Peter Hain, the Northern Ireland Secretary, is attracting the most financial backing. He has declared a £15,000 donation from the computer tycoon Bill Bottriell, received two other significant donations and is expecting dozens more.

Mr Johnson, Mr Hain and Mr Cruddas, the MP for Dagenham, all claim to have won the backing of more than the 44 MPs needed to get formally nominated under party rules. Mr Johnson has a big lead, with between 70 and 100 pledges of support.

Mr Johnson’s candidacy receives a significant boost today with the public support of seven respected ministers, representing different wings of the party. James Purnell, the Pensions Minister, said in a statement: “Alan Johnson has emerged as one of the major players of this Labour Government. His commitment to equality, improving social mobility and reforming public services combined with extensive experience inside and outside Westminster make him the best person for the deputy job.”

The endorsement of Mr Purnell, tipped as a cabinet minister in the near future, is seen as particularly significant, because he is unlikely to have gone public with his support if he knew that Gordon Brown, the Chancellor and almost certainly the next Labour leader, was opposed to Mr Johnson.

The other ministers declaring their support for Mr Johnson are Malcolm Wicks (Science), Ivan Lewis (Health), Phil Hope (Education), Tom Harris (Railways), Beverley Hughes (Children) and Gerry Sutcliffe (Prisons), who is managing Mr Johnson’s campaign.

The three other campaigns – Hilary Benn, the International Development Secretary, Harriet Harman, the Constitutional Affairs Minister, and Hazel Blears, the Labour Party chairman – are all said to be struggling to win enough nominations to get their names on the ballot paper.

Mr Benn, who was the early favourite, has the help of the Hilton and Hilton public relations company and the Campaign Company lobbying group, but he is not getting the backing of many colleagues in the House of Commons. “Hilary is finding it far harder to get the names than he thought he would,” one insider said.

Ms Harman has polling showing that Labour is most likely to win a fourth term in power if she becomes deputy leader, but has the public support of 21 MPs, including Patricia Hewitt, the Health Secretary. Ms Blears has not even officially announced her campaign because of concerns that she might be accused of abusing her position as party chair.

Having swept up at Westminster, Mr Johnson is now setting up a campaign headquarters in Leeds to win support across the country. Under the party’s rules, the vote is split three ways between 200,000 Labour party members, 3.2 million trade union members and 371 MPs and MEPs.

With such a varied electorate, the campaigners are adopting different strategies to win the ultimate prize.

Mr Cruddas may not have as much support as Mr Johnson among MPs, but he has positioned himself as the voice of ordinary trade unionists and party activists against the party establishment and has been energetically touring the country to win grassroots support.